Honestly having used Metagross myself just before the BW2 update, I have to say I was massively disappointed.
The biggest issue I found with it wasn't its speed so much as it was the fact that it's STAB is steel type, which is quite frankly just plain awful. Way too many Pokemon could switch in with impunity on him and threaten back (due to Metagrosses Fire and Ground Weaknesses). Metagross' coverage options aren't good enough to deal with enough of his switch ins to make him useful, and he always ends up with serious 4MSS. He never got Stealth Rock up reliably for me either because of the aforementioned weaknesses, which made staying in with him without attacking for a turn a bit of a stupid move. Maybe a CB set would work better with some more power, but CB and specs sets aren't exactly common these days unless the user is so powerful with it that they can plow through resists (like Tornadus, Terrakion, Dragonite, etc... Scizor is a unique exception obviously), and Metagross falls just short of that.
So yeah, I can see why he gets so low usage in OU; his offensive typing really bites him in the ass and his defensive capabilities aren't enough to make up for it. Shame really, I always liked Metagross.
This is a really good post that sums up my feelings towards Metagross. I hate to say it, but Metagross isn't very good in BW. I hate how it easily invites things like Skarmory & Forretress (and the occasional Ferrothorn) that set up all over your team, turning it into a liability. My favorite use for Metagross in BW was its ability to trick iron ball onto to Skarmory in order to remove its flight immunity but other than that I can't see legitimately using it over other Steel-types.
I'm actually surprised it hasn't dropped to UU. It's got some fiercely loyal fans.